1. Field of the Invention
The present invention relates generally to transparent conductive oxide (TCO) coatings for use in photovoltaic (PV) devices and methods of making the same. More particularly, the invention relates to improving the optical and electrical properties of transparent conductive oxide thin films and methods of making the same.
2. Discussion of the Background
All United States patents referred to herein are hereby incorporated by reference in their entireties. In the case of conflict, the present specification, including definitions, will control.
As the global population continues to grow, so does the demand for energy and energy sources. Fossil fuel consumption has seen steady increases during the last century, as expected for an energy thirsty global population. It was estimated that in 2004, 86% of human-produced energy came from the burning of fossil fuels. Fossil fuels are non-renewable resources and fossil fuel reserves are being depleted quicker than they can be replaced. As a result, a movement toward the development of renewable energy has been undertaken to meet increased demand for energy. Over the last ten to twenty years, there has been an increased focus on developing technology to efficiently harness energy from alternative sources, such as solar, hydrogen and wind energy to meet the increased global demand.
Of the alternative sources, the sun is considered the most abundant natural resource, with an infinite supply of energy showering the Earth on a daily basis. Numerous technologies exist that are directed to capturing the sun's light energy and converting it into electricity. A photovoltaic (PV) module represents such a technology and, to date, has found many applications in areas such as remote power systems, space vehicles and consumer products such as wireless devices.
PV modules are known to incorporate thin films. Thin film photovoltaics require a transparent front conductor, usually a thin film. The most common conductive thin films used are transparent conductive oxides such as fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO) and indium tin oxide (ITO). The main function of a TCO is two-fold. First, the TCO allows light to pass through to an active light absorbing material beneath it. Second, the TCO serves as an ohmic contact to transport photo-generated charges away from the light absorbing material. Such TCOs are desirable for all types of photovoltaic and solar modules, and are especially desirable for photovoltaic and solar modules based on amorphous silicon.
Improving the efficiency of PV devices incorporating TCO thin films typically has been limited by a number of factors. One of these factors is the inherent limitation of the TCO thin film conductivity, whereby it has been found that attempts to increase the conductivities are hampered by a simultaneous decrease in light transmission through the TCO thin film, which, in turn, decreases the efficiency of the PV device. The benefits of small improvements in photovoltaic efficiency can accrue over the life of the module and enhance financial return. Improvements in TCO optical and electrical properties can add to photovoltaic efficiency.
While photovoltaics have found many uses, there still exists a number of obstacles to overcome before photovoltaics can be competitive with traditional fossil fuels. Along these lines, cost and light-to-electricity conversion efficiency represent two of the obstacles preventing photovoltaics from being competitive with fossil fuels.
Most PV modules that exist today are based on silicon. Glass is omnipresent and, as such, provides an existing infrastructure for deployment of PV modules. One approach along these lines has been to adapt established glass production methods for later incorporation into a PV module. One such glass production method is the float-line method for producing float, or flat, glass.
Thin films on glass are desirable for numerous reasons. For example, architectural glass that is coated with a low emissivity coating possesses better insulating properties and is more energy efficient than architectural glass that is not coated. As a result of this desirability for thin films on glass, many methods exist for producing glasses coated with thin films. One of these methods is pyrolytic chemical vapor deposition, in which a metal containing species with micron and/or submicron thicknesses are deposited directly onto the glass surface. Such metal containing species include, but are not limited to, metals, metal oxides, metal nitrides, metal oxynitrides and metal oxycarbides.
There are many methods that exist for manufacturing thin films on glass. One such method is known as the online method, defined generally herein as coating the glass ribbon during the glass production. With respect to TCO thin films on glass incorporated into PV modules, there is still a need for methods that allow for improvements in the optical and electrical properties of these films before, during and after deposition on a glass substrate.
U.S. Pat. No. 7,259,085 discloses a method of making a metal oxide thin film wherein hydrogen chloride (HCl) is added to a gas stream of starting materials used for deposition of the metal oxide thin film. The purpose of HCl addition to the starting material gas stream is to prevent the starting materials from undergoing chemical reaction prior to reaching a surface of a glass substrate. This, in turn, leads to the ability to manufacture metal oxide thin film coatings having relatively uniform film thicknesses over a wide area and a long time at high film deposition rates (about 4500 nm/min or greater). It is disclosed that when metal oxide thin film coatings made by the methods of U.S. Pat. No. 7,259,085 are incorporated into photovoltaic devices, the likelihood of defects, such as pinholes, over the lifetime of such a photovoltaic device is reduced and the conversion efficiency of the photovoltaic device can remain high. However, there is no disclosure as to whether the addition of HCl to the starting material gas stream can lead to improved electrical conductivity of the metal oxide thin film coating after deposition of the metal oxide thin film.
U.S. Pat. No. 5,698,262 discloses a method of making a doped tin oxide film using hydrogen fluoride (HF) as a dopant source in order to obtain low and uniform sheet resistance of the metal oxide thin film. The metal oxide thin film layer therein is made from the starting materials: dimethyltin dichloride, oxygen, water, hydrogen fluoride and helium. It is disclosed in U.S. Pat. No. 5,698,262 that the fluorine doped tin oxide coatings made therein exhibit lower sheet resistance and improved uniformity in sheet resistance over the coated surface of the glass. However, the fluorine doped tin oxide coating disclosed therein still suffers from hazing. With a thickness of the fluorine doped tin oxide coating of only 320 nm, the size of the crystal grains therein would be limited, thereby preventing large irregularities of the film surface, which prevents a high haze ratio from being achieved.
U.S. Pat. No. 6,551,715 discloses a glass sheet with a transparent conductive doped tin oxide film wherein the tin oxide film is processed in such a way that a decrease in the conductivity of the tin oxide film is suppressed after a heat treatment of the tin oxide film. It is disclosed in U.S. Pat. No. 6,551,715 that conventional transparent conductive films of about 200 nm thicknesses disposed on glass substrates, when heat-treated, usually undergo a considerable sheet resistance increase. In order to minimize this sheet resistance increase, the thickness of the tin oxide film is set to at least about 400 nm. It was found that the decrease in sheet resistance by increasing the tin oxide film thickness tends not to be affected by heat treatment in air.
Thus, there remains a need in the art for transparent conductive oxide thin film layers that can overcome the above-noted problems of prior art films. In particular, there is a need in the art for TCO thin films having improved electrical and optical properties and for methods of making them.